1. Field of the Invention
The present invention relates generally to a private branch exchange system, and in particular, to a method for providing an internal subscriber, i.e., an extension line user of the private exchange system, with an outgoing call reservation service for a specified external (central office line) subscriber when the external subscriber's line is busy, when making a telephone call over the central office (C.O.) line.
2. Description of the Related Arts
In general, a private branch exchange (hereinafter referred to as “PBX”) switches an incoming call from a central office line or an extension line to a specified extension line in response to a call switching request, or vice versa. In that regard, a PBX system generally includes a PBX, subscriber terminals (i.e., extension telephones), and a control terminal. The PBX itself can have various configurations, but typically includes at least a controller, a switching circuit, a memory, and a tone generator. Conventionally, for the purpose of making a telephone call to any central office line subscribers such as other private subscribers or a telephone information service, an internal subscriber (subscriber A), i.e., an extension line user of the private branch exchange system, needs to make a seizure of a central office line (also referred to as C.O. line) using his own extension telephone, and dial up a telephone number of an intended called party (subscriber B). At this time, if the called party, i.e., the line of subscriber B, is busy, then the calling party, i.e., the subscriber A, should dial up the same telephone number again after a while or probably use a re-dialing function provided by the extension telephone or the private exchange system in order to make the intended outgoing call to subscriber B.
In the meantime, when subscriber B is busy on the phone, an automatic re-dialing function by which a re-dialing is carried out repeatedly with a specified time interval up to a predetermined number of retries of dialing may be utilized. Use of such an automatic re-dialing function would remove the needs for subscriber A to manually repeat dialing on the same telephone number until subscriber B can be reached, thereby eliminating subscriber A's inconvenience considerably when making an outgoing call via a central office line to subscriber B.
In general, a redial calling service provided with the PBX itself allows its extension subscriber to make a call reservation for a desired outgoing calling via an office line when the called party's line cannot be rung due to it being busy. Thus, after completion of the called party's busy state, the redial calling service allows the subscriber to be connected automatically to the called party by actuating the reserved outgoing calling so as to establish a speech channel between both parties concerned. Thus, the extension subscriber would be automatically called via the reserved office line to the called party without having to re-dialing the called party's phone number repeatedly, when the called party's telephone is no longer busy.
However, when the amount of outgoing calls to a specified called party is overcrowded, even the use of a re-dialing service by the subscriber often fails to make a successful connection to the called party, owing to the continuous busy state of the called party's telephone line occupied by other calling parties. This is because many other calling parties are actually trying to call the same called party while the re-dialing by the subscriber is being served, even though the called party is no longer using his/her telephone. Thus, under this situation, a connection may have been made to between the called party and one of other calling parties just after the called party hangs up upon completion of his/her conversation with another party. This situation is very annoying to the subscriber and often causes him/her a waste of time, eventually resulting in failure of calling the called party, as a worst case scenario.
As aforementioned, when the amount of outgoing callings to a specified called party is overcrowded, even the use of a redial service by the calling party gives a relatively low probability of success in establishing a call connection to the called party owing to the continuous busy state of the called party's single telephone line occupied by other calling callers. Thus, it often takes a lot of time in successfully making a desired call connection, even leading to an eventual failure in establishing the call connection.
To solve this problem, when subscriber A tries to reach subscriber B, several extension subscriber's telephones may be employed together to make the desired outgoing call connection at the same time. However, this case still not only causes the waste of more time but also the attempt to call subscriber B more complex. Further, a critical problem arises, upon call connection with subscriber B via another extension subscriber's telephone, in attempting to transfer the call to subscriber A's own telephone from the other extension subscriber's telephone, or subscriber A must use the other extension subscriber's telephone, which would be an inconvenience not only to subscriber A but also to the other extension subscriber.